The Great Wall roughly divides China from the Inner Mongolia. If was very useful not just to protect China from the Mongolians but also as a border control and allowing impositions of duty on transports along the Silk Road.
The Great Wall of China consists of many fortifications made of stone, brick and wood. It was built to protect the Chinese empire against the Mongolian tribes. The Great Wall stretches from Shanhaiguan in the east to Lop Lake in the west and is about 8850 km long.